1. Field of the Invention
The present invention relates to a digital audio watermark, and more particularly, to an apparatus and method of inserting and detecting watermark information within a bit steam in a high quality audio encoding process.
2. Discussion of the Related Art
Watermarking refers to embedding secret information called “watermark” into a medium such as video, image, audio and text. The embedded watermark information can be extracted with limitation to those who know it. Medium having a watermark is recognized by common users to be the same as a general medium.
Specifically, a digital medium brings about a new issue of a copyright protection, due to its advantage comparing with an analogous medium, in which access, transmission, edition and keeping are easy and data degradation is not caused at the time of data distribution through an electric wave or a communication network. Digital watermarking is noted as means for protecting a copyright.
The digital watermarking is not only used for inserting information to distinguish a proprietor to protect the copyright, but is also used for inserting control information for anti-copy, distribution confirmation, a broadcasting monitoring and the like or used for inserting information such as presentation time control information, synchronization (lip-sync), contents information and song words into a real time medium such as audio, video and the like and transmitting the inserted information.
As such, the digital watermarking has a different characteristic depending on a variety of usage purposes, but imperceptibility and robustness are no doubt essential.
The imperceptibility being the most basic requirement means that an original medium and a watermark inserted medium are not distinguished from each other when users view or listen to them.
The robustness means that even though the watermark inserted medium is deformed such as filtering, compressing, noise addition and degradation required for distribution and transmission, the inserted watermark should be preserved.
Specifically, a watermark for the copyright protection and the anti-copy should be robust so that it can cope with an intentional attack intended to eliminate the watermark. Meanwhile, a watermark for forgery-free is easily extinguished when it is deformed or manipulated.
Further, a watermark for embedding additional information such as presentation time control information, lip-sync, contents information and song words into the medium has a relatively low robustness against the intentional attack or distortion.
A general method of the digital watermarking is illustrated in FIG. 1.
As shown in FIG. 1, watermark data is embedded into a digital medium (audio, video, image, text and the like) by using a watermark insertion system 1. At this time, a secret or public key for security can be additionally used depending on a watermarking algorithm.
After that, the inserted watermark can be extracted from a watermark inserted medium by using a watermark extraction system 2. At this time, an original medium can be required depending on the watermark algorithm, and the decoding can be also performed using only the public key required at the time of inserting.
A system not requiring the original medium in a watermark extracting process is called “blind watermarking”.
Among watermarking methods, an audio signal watermarking method is variously exemplified such as a Least Significant Bit (LSB) encoding method, an echo hiding method, and a spread spectrum communication method and the like.
In the LSB encoding method, least significant bits of a quantized audio sample are deformed to insert desired information. The LSB encoding method uses a characteristic in which the deforming of the least significant bit of an audio signal does not almost have influence on a sound quality. The LSB encoding method has an advantage in that insertion and detection is simply performed and the sound quality is less distorted, but has a drawback in that it is vulnerable to signal processing such as loss compression or filtering.
Further, in the echo hiding method, an inaudible echo is inserted into an audio signal. That is, the echo hiding method inserts and encodes an echo with a different time delay into the audio signal, which is subdivided at a predetermined interval, depending on binary watermark information to be inserted. In a decoding process, binary information is decoded by detecting an echo time delay at each of subdivided durations. In this case, the inserted signal is not a noise, but is the audio signal itself having the same characteristic as an original signal. Therefore, even though the inserted signal is heard, the inserted signal is not recognized as a distorted signal. The inserted signal is rather expected to provide a better tone. Accordingly, the echo hiding method is suitable to a high quality audio watermarking, but has a disadvantage in that since the detecting is performed using a Cepstrum operation, an operation amount of decoding is very high, and in case where the synchronization for the duration to be subdivided at a time-domain is missed, the decoding is not performed.
Further, the spread spectrum communication method is a typical watermarking method, which is popularized for video watermarking and most studied even for audio watermarking. In the spread spectrum communication method, an audio signal is transformed into a frequency through a discrete Fourier transformation and then, binary watermark information is spectrum-spread to a PN (Pseudo Noise) sequence to insert spread information into the frequency-transformed audio signal. An inserted watermark can be detected using a correlator by using a high auto-correlation characteristic of the PN sequence, and have a characteristic of robustness against interference and an excellent encryptability. On the contrary, the spread spectrum communication method has a drawback in that a sound quality is deteriorated, an operation amount of insertion and detection is very high, and a compression encoding is incomplete in case where the watermark is inserted with a large energy to improve robustness.
As such, summarizing the conventional audio watermarking, the conventional audio watermarking has a drawback in that its implementation method is complex since the watermark information is generally inserted into the original signal before the original signal is compressed and decoded, and accordingly the operation amount is required much and the original signal is easily deformed when it is compressed.